Various open loop and closed loop systems for the extraction of geothermal energy have been proposed. Several patents and patent publications describing these may include, for example, U.S. Pat. Nos. 3,824,793, 3,935,102, 3,951,794, US20100272515, U.S. Pat. Nos. 8,281,591, 7,849,690, US20130192816, JP2000161198, U.S. Pat. Nos. 8,650,875, 6,668,554, US20110048005, U.S. Pat. No. 7,059,131, US20110067399, US20120144829, US20130192816, US20120174581, US20100180593, and US20070245729, among others.
Typical open loop systems rely upon a fluid, such as water or carbon dioxide, to fracture and traverse through a subterranean formation, where direct heat transfer with the surrounding rock increases the temperature of the fluid. The heated fluid is then recovered from the formation and used, for example, to generate electricity by extracting heat from a portion of the heated fluid or expanding the fluid.
Existing open loop systems proposed to utilize rock formations having relatively high permeability. Commercial geothermal power production projects access geothermal energy within high permeability areas, such as fault zones, within the brittle zone. These projects extract energy from geothermal reservoirs using geothermal brine with open loop systems. The geologic resource temperature is typically between 150° C. and 300° C. The minimum necessary permeability for such projects is normally about 10−6 m2. The practical effect of this is that commercial projects are restricted to a small percentage of the brittle zone in areas in which temperature, permeability, and geothermal brine are all present in sufficient degree. Such geothermal systems are also constrained by depth because permeability rapidly decreases with depth, which suggests that there is a vertical limit to which hydrothermal projects can extend before the permeability becomes too low for projects to have adequate flow to sustain operations. In other words, hydrothermal projects cannot simply go “deeper” and/or “hotter” to improve heat extraction.
Environmental concerns and losses to the environment, among other factors, resulted in the consideration of naturally low permeability rock to “contain” a fluid, such as proposed in U.S. Pat. No. 6,668,554. The permeability of the low permeability rock, however, was locally increased to provide for mass transfer through the formation and recovery of the heated fluid. Even in these systems, a need for permeable rock is taught as necessary.
Typical closed loop systems include a fluid loop that passes through a hot subterranean formation. Even in such systems, it is generally considered necessary to fracture the formation to provide for both convective and conductive indirect heat transfer to the fluid contained within the closed loop system. Increased permeability of the formation near the closed loop system is generally considered by one skilled in the art as necessary for convective currents to form within the formation, enhancing the heat transfer. As a result of the general consensus in the art, only resources that were generally considered suitable for open loop systems were considered suitable for the closed loop systems, essentially envisioned as an overlapping resource selection.